The principles that define usability and contribute to consistent accuracy of muzzle loading firearm projectiles have not changed much since the late 16th century. Firearm and projectile designers have worked continuously to minimize the loading efforts of muzzle loading projectiles while at the same time attempting to develop ideas that would consistently assure an effective gas seal and engagement of the projectile with the rifling of the firearms barrel. If the projectile loading efforts are too high or inconsistent the projectile will not be loaded in contact with the powder charge leading to inconsistent load points and possibly dangerous air gaps between the projectile and the powder charge resulting in unacceptable accuracy. If upon ignition of the powder charge the projectile does not seal the propellant gases or engage with the barrel rifling, rotary motion will not be imparted to the projectile and it will not stabilize in flight, also causing unacceptable accuracy. Over the course of the last three centuries, four major types of projectiles have evolved to accommodate the projectile requirements of muzzle loading firearms covering the spectrum from hand held firearm to the mid 19th century cannons.
The oldest form of muzzle loading projectiles are the all lead round ball or conical bullet wrapped in a material that fills the space between the bore and groove diameters of the firearm barrel. The wrapper serves three purposes namely: it fills the void between the bore size bullet and the groove diameter of the barrel creating an effective gas seal; it also is the mechanism that engages the projectile with the barrel rifling to create the rotary motion necessary to stabilize the projectile and create a predictable flight trajectory; and it also prevents movement of the projectile once seated on the powder charge regardless of barrel position. A number of different materials have been utilized for this wrapper or gas seal including cloth, paper, or more recently plastic. This style of projectile was used extensively for hunting, target and military applications through the 19th century.
The most recent refinement of the wrapped or encased bullet was developed and refined over the last 30 years and is defined as a sabot. The sabot is basically a plastic tube with a partition in the middle that separates the bullet from the powder charge. The portion of the sabot towards the powder charge is cupped with thin exterior walls that act as a gas seal when the powder charge is ignited. The walls of the cylinder that encase the bullet are thicker than the cloth or paper patch and are slit in multiple locations through the area that contains the bullet to allow the sabot to release and fall away from the bullet once the two have exited the barrel muzzle. The increased wall thickness of the sabot allows for bullets up to two caliber sizes smaller than a full bore projectile that would normally be used. An example of this would be a sabot with an inside diameter of 44 caliber or 0.429 inches in diameter and an outside diameter of 50 caliber or 0.510 inches in diameter allowing a 44 caliber bullet to be fired in a 50 caliber firearm. Sabots have been developed for 54, 50, and 45 caliber firearms with 50 being the most popular. The ability to fire sub bore projectiles accommodates a number of disadvantages that exist with the current full-bore projectiles or bullets. The major advantage that the sub caliber bullet has over the full-bore projectile is that significantly higher velocities can be achieved with a common powder charge. The sub bore bullets will typically be much lighter with better ballistic efficiencies than the full bore projectile. The higher velocities and better ballistic profile contribute to significant flatter trajectories and similar impact energies at normal hunting distances.
The trend in recent years has been to use the sabot technology to drive light bullets of heavy construction to velocities approaching those typified by center fire rifles. The features of the sabot that allow the use of light sub bore bullets also contribute to its limitations. As the projectile velocities approach 2,000 fps, the propellant pressures necessary to accelerate the projectile to this velocity exceed the physical limitations of the plastics that the sabots are composed of. In addition, this problem is exacerbated as the environmental temperatures exceed 75° F. degrees and the elongation of the plastic increases with the increase in temperature. As the physical properties of the plastics are exceeded, accuracy deteriorates quickly due to the plastic of the sabot coating the inside of the barrels and the disintegration of the pressure cup at the base of the sabot. Sabots are often hard to load due to the number of variables that must be accounted for between the sabot, bullet, and barrel and associated pressures. Another deficiency of sabots is that it is often necessary to swab the bore of the firearm between firing sequences with a damp and then dry wad to prevent the build up of the expended powder residue from the previous firing. If the barrel is not swabbed between shots, accuracy will deteriorate quickly due to the build up of residual matter left from ignition of the previous powder charge altering the frictional characteristics between the sabot and the firearm barrel. An additional draw back to the sabot style of projectile is that it is not legal for use for big game hunting of species larger than deer in most of the western United States.
In the early to mid 19th century, considerable development work was focused on the development of a full bore elongated lead bullet that could be easily loaded but would expand to seal and engage the barrel rifling. The designs typically were composed of an elongated lead bullet with multiple grooves and hollow base. The grooves may or may not have been filled with a lubricant the purpose of which was to allow for ease of loading and an attempt to keep the residual powder fouling build up soft from the previous firing sequence. The only major difference between the mid 19th century and present day bullet designs of this style is that one of the major diameters of the circumferential grooves of the bullet is larger than the bore diameter of the barrel. The modern designers have increased the ring diameter to prevent the bullet from shifting within the barrel regardless of barrel position. The purpose of the hollow skirt is to act as a gas seal when the powder charge is ignited expanded to the barrel groove diameter and a mechanism to impart spin to the bullet as it passes through the barrel. The all lead full bore projectile's are typically heavy for caliber due to their composition which limits their effective hunting range to 125 yards or less. These projectiles also require that the firearms barrel be swabbed between firings to ensure loading efforts do not become excessive due to fowling building up from the previous ignition sequence. This type of projectile or bullet will only function correctly if composed of lead. Currently, within the United States, there is a movement to ban the use of lead in firearm projectiles. Legislation to prevent the use of lead for waterfowl hunting was successfully passed in the United States in the late 20th century and is presently being pursued for firearms in the regions of California inhabited by Condors.
The final type of major projectile developed for muzzle loading firearms is a full bore thin skirted bullet. Two variations of this style of projectile have evolved, the first of which was developed in the mid 19th century for use in the civil war cannon. Examples of this design can be reviewed in U.S. Pat. No. 15,999 issued to John B. Reed and U.S. Pat. No. 33,100 issued to R. P. Parrott. The body of the projectiles was typically composed of cast iron or steel with a hollow thin iron or brass/bronze skirt secondarily attached. The outside diameter of the projectile is slightly smaller than the bore diameter of the barrel it is to be fired in. Upon detonation of the powder charge, the hollow skirt of the projectile expands to act as a gas seal and engage the rifling of the barrel imparting rotary motion and stabilizing the projectile in flight. The second variation of this idea can be viewed in U.S. Pat. No. 5,458,064 issued to R. M. Kerns. This design was developed for modern muzzle loading firearms and uses a thin plastic skirt attached to the base of the bullet by a small extruded stub at the posterior of the bullet. The outside of the diameter of the bullet is slightly smaller than the bore diameter of the barrel to allow for ease of loading. Upon ignition of the powder charge, the plastic skirt expands and acts as a gas seal. The bullet is composed of a soft lead which upon detonation of the powder charge expands to engage the rifling of the barrel to impart rotary motion to the projectile. Due to the number of variables involved between the bullet and the barrel, it is difficult to depend on the predictability of this style of bullet to expand or obturate to the groove diameter of the barrel to ensure that rotary motion is imparted. Temperature, pressure, and rate of ignition of the powder charge all play a role of differing levels depending on the environmental conditions at the time. Additionally, the plastic skirt for this style of projectile will have the same limitations from a velocity perspective as that seen with the sabot style. The sabot and the gas check on the Kerns style bullet both can create small air pockets between the projectile and powder charge, which can retard the rate of ignition of the powder ignition leading to inconsistent projectile velocities and accuracy.
It is therefore a primary object of the present invention to provide a projectile having in combination a multi diameter hollow base solid copper bullet filled with an expansion plug so that when utilized in conjunction with a gas check member, the bullet has the ability under normal muzzle loading firearm propellant pressures to expand the shank portion of the bullet filled by the expansion plug to engage the barrel rifling and impart rotary motion to the bullet.
It is also an object of the present invention to provide a projectile with a multi diameter shank bullet so that the majority of the bullet can be easily loaded within the bore of the intended firearm but has the ability to self center when the projectile is fully loaded within the bore of the firearm.